Guarantor Financial Information - Condensed consolidating statement of income and comprehensive income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Revenues
Premiums Earned, Net	$ 859,780		$ 752,770
Net investment income	66,994		65,672
Net realized gains	19,697		58,340
Net impairment losses recognized in earnings	(2,971)		(2,246)
Other underwriting income	1,582		538
Equity in net income of investment funds accounted for using the equity method	3,253		13,823
Other income (loss)	(2,104)		1,244
Total revenues	946,231		890,141
Expenses
Losses and loss adjustment expenses	436,240		399,403
Acquisition expenses	160,342		127,592
Other operating expenses	145,799		120,183
Interest expense	14,404		5,898
Net foreign exchange losses (gains)	6,563		(24,264)
Total expenses	763,348		628,812
Income (loss) before income taxes	182,883		261,329
Income tax expense (benefit)	(3,738)		(4,853)
Income Loss Net Of Income Taxes And Before Equity In Net Income Of Subsidiaries	179,145		256,476
Income (Loss) from Subsidiaries, Net of Tax	0		0
Net income	179,145		256,476
Net (income) loss attributable to noncontrolling interests	3,355	[1]	0	[1]
Net income available to Arch	182,500		256,476
Preferred dividends	(5,484)		(5,484)
Net income available to Arch common shareholders	177,016		250,992
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	231,255		199,022
Reportable Legal Entities | ACGL (Parent Guarantor)
Revenues
Premiums Earned, Net	0		0
Net investment income	0		0
Net realized gains	0		0
Net impairment losses recognized in earnings	0		0
Other underwriting income	0		0
Equity in net income of investment funds accounted for using the equity method	0		0
Other income (loss)	0		0
Total revenues	0		0
Expenses
Losses and loss adjustment expenses	0		0
Acquisition expenses	0		0
Other operating expenses	10,307		9,942
Interest expense	5,853		5,820
Net foreign exchange losses (gains)	0		0
Total expenses	16,160		15,762
Income (loss) before income taxes	(16,160)		(15,762)
Income tax expense (benefit)	0		0
Income Loss Net Of Income Taxes And Before Equity In Net Income Of Subsidiaries	(16,160)		(15,762)
Income (Loss) from Subsidiaries, Net of Tax	198,660		272,238
Net income	182,500		256,476
Net (income) loss attributable to noncontrolling interests	0	[1]	0
Net income available to Arch	182,500		256,476
Preferred dividends	(5,484)		(5,484)
Net income available to Arch common shareholders	177,016		250,992
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	231,255		199,022
Reportable Legal Entities | Arch-U.S. (Subsidiary Issuer)
Revenues
Premiums Earned, Net	0		0
Net investment income	0		0
Net realized gains	0		0
Net impairment losses recognized in earnings	0		0
Other underwriting income	0		0
Equity in net income of investment funds accounted for using the equity method	0		0
Other income (loss)	0		0
Total revenues	0		0
Expenses
Losses and loss adjustment expenses	0		0
Acquisition expenses	0		0
Other operating expenses	977		777
Interest expense	6,514		3
Net foreign exchange losses (gains)	0		0
Total expenses	7,491		780
Income (loss) before income taxes	(7,491)		(780)
Income tax expense (benefit)	2,788		235
Income Loss Net Of Income Taxes And Before Equity In Net Income Of Subsidiaries	(4,703)		(545)
Income (Loss) from Subsidiaries, Net of Tax	22,152		18,482
Net income	17,449		17,937
Net (income) loss attributable to noncontrolling interests	0	[1]	0
Net income available to Arch	17,449		17,937
Preferred dividends	0		0
Net income available to Arch common shareholders	17,449		17,937
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	17,220		6,152
Reportable Legal Entities | Other ACGL Subsidiaries
Revenues
Premiums Earned, Net	859,780		752,770
Net investment income	77,001		72,322
Net realized gains	19,697		58,340
Net impairment losses recognized in earnings	(2,971)		(2,246)
Other underwriting income	1,582		538
Equity in net income of investment funds accounted for using the equity method	3,253		13,823
Other income (loss)	(2,104)		1,244
Total revenues	956,238		896,791
Expenses
Losses and loss adjustment expenses	436,240		399,403
Acquisition expenses	160,342		127,592
Other operating expenses	134,515		109,464
Interest expense	12,044		6,725
Net foreign exchange losses (gains)	5,741		(12,772)
Total expenses	748,882		630,412
Income (loss) before income taxes	207,356		266,379
Income tax expense (benefit)	(6,526)		(5,088)
Income Loss Net Of Income Taxes And Before Equity In Net Income Of Subsidiaries	200,830		261,291
Income (Loss) from Subsidiaries, Net of Tax	0		0
Net income	200,830		261,291
Net (income) loss attributable to noncontrolling interests	3,355	[1]	0
Net income available to Arch	204,185		261,291
Preferred dividends	0		0
Net income available to Arch common shareholders	204,185		261,291
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	252,119		215,329
Consolidating Adjustments and Eliminations
Revenues
Premiums Earned, Net	0		0
Net investment income	(10,007)		(6,650)
Net realized gains	0		0
Net impairment losses recognized in earnings	0		0
Other underwriting income	0		0
Equity in net income of investment funds accounted for using the equity method	0		0
Other income (loss)	0		0
Total revenues	(10,007)		(6,650)
Expenses
Losses and loss adjustment expenses	0		0
Acquisition expenses	0		0
Other operating expenses	0		0
Interest expense	(10,007)		(6,650)
Net foreign exchange losses (gains)	822		(11,492)
Total expenses	(9,185)		(18,142)
Income (loss) before income taxes	(822)		11,492
Income tax expense (benefit)	0		0
Income Loss Net Of Income Taxes And Before Equity In Net Income Of Subsidiaries	(822)		11,492
Income (Loss) from Subsidiaries, Net of Tax	(220,812)		(290,720)
Net income	(221,634)		(279,228)
Net (income) loss attributable to noncontrolling interests	0	[1]	0
Net income available to Arch	(221,634)		(279,228)
Preferred dividends	0		0
Net income available to Arch common shareholders	(221,634)		(279,228)
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	$ (269,339)		$ (221,481)

[1] See Note 4.